The dopamine transporter, expressed almost exclusively on dopamine neurons, is a sensitive marker for physiological and pathological changes in dopamine neurons In Parkinson's disease, a view of the status of dopamine neurons in living brain has contributed significantly to our understanding of the natural progression of the disease and the relationship between disease severity and dopamine neuron depletion Imaging of dopamine neurons can identify populations that will benefit from treatments designed to arrest disease progression or promote regeneration of dopamine neurons Dopamine transporter imaging has extended to other states with suspected abnormalities in dopamine neurons (e g Huntington's disease, methamphetamine abuse) In 1989-90, this laboratory first identified potent phenyltropane dopamine transport inhibitors (e g CFT or WIN 35,428) as promising imaging agents for PET (positron emission tomography) or SPECT (single photon emission computed to mogr aphy) imaging of the transporter [11C]CFT ([11C]WIN 35,428) and its analog [123I]or [11C]altropane have progressed to clinical trials 99mTechnetium (99mTc), is the most widely used for imaging of peripheral tissues as it is the least technical challenge However, Tc-based probes present a significant challenge for brain imaging because the metal chelate needed to attach the isotope to a probe results in low brain penetration of the probe We now report a novel99mTc imaging agent, O-1505-T which penetrates the brain at relatively high concentrations In vitro, the precursor O-1506 displayed high affinity for the dopamine transporter (IC50 2 05 nM) and low affinity for the serotonin transporter (IC50 497 nM), resulting in a selectivity ratio > 240 Four normal and two MPTP-treated monkeys were injected with 10-25 mCi of O-1505T and serial SPECT images were acquired over 2 h The images were reconstructed using a filtered back projection algorithm and striatal-cerebellar ratios we re calculated In normal animals, accumulation of O-1505T in the dopamine-rich striatum was rapid and peak levels were achieved within 30 min Accumulation was nearly completely displaceable with unlabeled CFT (1 mg/kg) but was not affected by a similar dose of the serotonin transporter-selective drug citalopram The striatal:cerebellar ratio fell from 2 5:1 to 1:1 in MPTP-treated monkeys Blood clearance of the probe was rapid and approached zero by 60 min The results demonstrate that O-1505T is superior to the previously reported agent technepine It is a suitable SPECT ligand for imaging dopamine neurons because it combines the critical characteristics of 1 Facile labeling with 99mTc; 2 High striatal:cerebellar ratio; 3 High selectivity for dopamine over serotonin transporters; 4 Favorable radiation dosimetry; 5 Pharmacokinetics well matched to the physical half-life of 99mTc This study provides further evidence of the feasibility of developing technetium-labeled compound s to image brain receptors and transporters PUBLICATION Madras BK Imaging the dopamine transporter a window on dopamine neurons Advances in Neurodegenerative Disorders 1998 (J Marwah and H Teitelbaum, eds ) Vol 1 Parkinson's disease, pp 229- 253 Prominent Press, Scottsdale, AZ